In modern electrical systems as used in houses, offices and other buildings, electricity is delivered by cabling to various points around the building at which there are electric sockets into which can be plugged the appliances to be used powered by the electricity supply, and switches to control the power to those sockets. Moreover, in a modern building there will be a technically similar system for providing communication channels—for telephone, television and computer. Each of the sockets will usually be of the type having an apertured terminal-carrying socket plate mountable on a recessed socket box itself flush mounted within a cavity in a wall. The apertures lead to the socket's terminals, disposed on the back of the plate and thus out of harm's way inside the box, and are shaped and sized to match the contact pins of the plug for which the socket is designed.
Another type of socket commonly encountered, especially in electric power circuits, is the “extension socket”, a group of three or more individual sockets formed in a box-like carrier and all connected by a single lead to a plug that plugs into a single wall-mounted socket (in this way a single such wall socket can, within reason, be “converted” into a plurality of sockets). Such an extension socket device is normally the appropriate plurality of apertured terminal-carrying socket plates all mounted on a socket box; in principle, therefore, it is little different from a wall-mounted socket.
There are many reasons why it might be desirable to provide a cover over a socket, or over a plug when in place in that socket. One is that empty sockets seems to exert an irresistible fascination for small children, who will try to poke their fingers, their toys, or any available long thin object—such as a knitting needle or a screwdriver—into the plug pin apertures in the socket; when applying such investigatory skills to an electric socket a persistent and inventive child may well be successful, but sadly the reward for such success is all too often death by electrocution. A second reason is that much modern equipment is designed to be plugged in and left on and connected all the time—falling into this category are refrigerators, televisions (and video recorders) and Fax machines at one extreme, and computers (and their networks) and medical life support systems at the other—and sometimes it may be a minor disaster if the device is disconnected, unplugged or turned off (by, for example, a cleaner looking for a socket into which to plug a vacuum cleaner). The problem, as always, is how to achieve a cover that is both effective, securely hiding the switch, socket or plug from an inquisitive child or a careless cleaner, and yet also relatively easy for an authorized person to remove in order to access the plug, or the socket and its switch.